Examples of a depolymerization method for reducing the viscosity of water-soluble cellulose ether include methods of using ozone or a volatile acid such as hydrogen peroxide or hydrogen halide. It is, however, presumed to be difficult to industrially materialize these methods because they use expensive reagents, leave reagents in the resulting products, or require special equipment.
Japanese Patent Application Unexamined Publication No. 61-264001 proposes a method for adjusting an oxygen amount in a reactor in preparation of alkali cellulose. Since the degree of polymerization of cellulose is reduced by a reaction (depolymerization reaction) between oxygen and cellulose in the presence of an alkali, cellulose ether having a lower viscosity can be obtained when an amount of oxygen is greater. This method is advantageous because it can use oxygen present in the air so that it does not require special equipment and in addition, the oxygen does not remain in the product thus obtained. In this method, a pulp is not depolymerized just by bringing the pulp into contact with oxygen. The pulp is depolymerized by reacting with oxygen in the reactor as the pulp is brought into contact with the alkali.
In general, there is a possibility of explosion during depolymerization after a step of etherification reaction in comparison with depolymerization during a step of preparing alkali cellulose so that the depolymerization after the step of etherification reaction has been avoided as much as possible. Accordingly, regarding depolymerization of water-soluble cellulose ether, it is the common practice to carry out the depolymerization in the step of preparing alkali cellulose in which a large amount of alkali is present and there is no risk of explosion by an etherifying agent.